Export obj with tga textures to obj with png textures using blender?
There is a way to do this in blender... baking.  But it is the most complicated way to do what you want.

The more efficient method, would be to open the images in Photoshop, Gimp, inkscape, or whatever, and just save them as png files.

Then, in blender you can just swap the textures of the material. Or instead of placing the .tgas place the newly created .png files. 

You should just be able to save the edits.